1. Technical Field
The present invention relates generally to satellite communication networks, and more particularly to a methodology for evaluating performance of a low earth orbit satellite (LEOS) system.
2. Related Art
Satellite communication systems are generally known to facilitate wireless communications across most areas of the earth's surface. Satellite communication systems may provide wireless coverage for mobile subscriber units in areas that are served neither by the public switched telephone network (PSTN) nor by cellular communication systems. The satellite communication systems may also provide a high bandwidth routing path for voice and data communications.
In a satellite communication system, at least one satellite operates from an orbit above the earth's surface. International telecommunication satellites typically operate from a geo-stationary orbit (GEO) that is approximately 36,000 kilometers above earth. Alternatively, telecommunication satellites have also been implemented in orbits closer to earth in low earth orbit satellite (LEOS) systems at an altitude from 700 kilometers to 2,000 kilometers or medium earth orbit satellite (MEOS) systems at approximately 10,000 kilometers.
In a LEOS system, a plurality of satellites orbit the earth and together provide coverage across most areas of the earth's surface. As contrasted to a GEO system in which a single satellite provides coverage over a substantial geographic area for all times, in a LEOS system the coverage area of each orbiting satellite changes over time. Thus, a first LEOS system satellite will provide coverage for a geographic area during a first time period while another LEOS system satellite provides coverage for the geographic area for a second time period, etc.
In a typical LEOS configuration, satellites are organized into orbital planes. A plurality of satellites orbits the earth in the orbital plane such that the satellites pass near the south pole and the north pole during their orbits. The satellites of each orbital plane are substantially uniformly distributed about the orbital plane and, as a whole, provide coverage for a geographic area corresponding to the orbital plane. By providing a plurality of orbital planes, each of which is separated from adjacent orbital planes by a separation angle, communications across a significant portion of the earth's surface are supported.
The number of orbital planes, the number of satellites in each orbital plane, the separation of the orbital planes, the altitude of the satellites in each orbital plane, and the inclination of each orbital plane characterizes the "constellation" of the LEOS system. The constellation essentially describes the relative positions and motion of the satellites in the LEOS system.
The LEOS system is called upon to service terrestrial communications between source and destination. Sources and destinations are devices that couple communications to the LEOS system. A source or destination may be a wireless subscriber unit, an earth station that couples the LEOS system to the PSTN or another communication network, or any other wireless device that couples to one of the satellites.
In a LEOS system, communications are not only coupled from sources and destinations to satellites, but they may be coupled between satellites as well via intersatellite links (ISLs). For the purposes of overall communication path length evaluation, each ISL may be referred to as a "hop." Each hop consumes LEOS system resources and adds delay to the communication. Thus, it is desirable to minimize the number of hops within the LEOS system when coupling communications between sources and destinations. However, the number of hops between a first satellite servicing a source and a second satellite servicing a destination depends upon system routing behavior and the positions of the source and destination. System routing behavior is based not only upon programmed permissible connections between satellites but upon the LEOS system constellation as well.
In designing and operating a LEOS system, it is important to evaluate the behavior of the system. However, because of the dynamic nature of the satellites in the LEOS system and the time varying ISL connectivity within the LEOS system, it is difficult to evaluate the system's behavior. Thus, there is a need in the art for a methodology for evaluating system performance of the LEOS system.